The present invention relates to an electrostatic chuck, and a method of and an apparatus for processing a sample using the chuck. The invention relates particularly to an electrostatic chuck suitable for electrostatically holding a sheet-like sample, such as a semiconductor substrate or a liquid crystal substrate, when the sample is processed or carried, and a method of and an apparatus for processing a sample using the chuck.
An example of a bipolar type electrostatic chuck using a pair of electrodes having different polarities, in the form of an electrostatically attracting apparatus having a pair of semi-circular or concentrically-circular shaped flat electrodes, is disclosed in Japanese Patent Laid-open No. 64950/1982. According to this document, by increasing the ratio between the area of the electrostatically attracting apparatus and the electrode area of the pair of flat electrodes, mounting a substance on the pair of flat electrodes through an insulator having a thickness of 50 to 200 xcexcm, and electrostatically attracting the substance by applying a voltage between the flat electrodes, it is possible to employ the electrostatically attracting apparatus to hold both a conductive substance and a conductive substance whose surface is covered with a thin insulating film to provide a stronger attracting force and to simplify the structure of the apparatus. The document also describes that the attracting force is maximized when the areas of the pair of positive and negative electrodes are equal to each to other. Also, another bipolar type electrostatic chuck is disclosed in Japanese Patent Laid-open No. 120329/1994.
The method of holding a sample, for example, a wafer using such an electrostatic chuck has advantages in that: (1) since the surface of a wafer to be processed is not mechanically contacted by the chuck, the wafer can be prevented from being contaminated by abrasive particles and the like; and (2) since the entire back surface of a wafer is fixedly attracted on the chuck, the camber of the wafer can be corrected, so that the contact of the wafer with an attracting surface of the chuck becomes more reliable when the wafer is finely processed by etching or the like, to improve the thermal conductivity of the wafer, thereby facilitating the temperature control of the wafer. For these reasons, the electrostatic chuck is being extensively used as a sample stage (referred to simply as an xe2x80x9celectrodexe2x80x9d) of a dry etching apparatus or a plasma processing apparatus, such as a CVD apparatus.
A bipolar type electrostatic chuck used for a plasma processing apparatus, in the form of an electrostatically attracting apparatus, for example, in Japanese Patent Publication No. 44747/1982. The document indicates that a larger attracting force can be obtained during plasma discharge by making the area of the positive electrode larger than that of the negative electrode, and that the attracting force in the case of generation of no plasma is maximized when the ratio between the areas of the positive and negative electrodes is set at 1.
Another disadvantage of the electrostatic chuck will be described below. In general, to remove a wafer from the electrostatic chuck after termination of the processing of the wafer, bar-like supports (generally, called xe2x80x9cpushersxe2x80x9d or xe2x80x9clift pinsxe2x80x9d) are lifted or pushed up from the interior of the electrostatic chuck for pushing up the wafer therefrom. The mechanism involving such bar-like supports is known, for example, from U.S. Pat. No. 4,565,601 or Japanese Patent Laid-open No. 252253/1994. However, in the case where there exists a residual attracting force between an electrostatic chuck and a wafer, if a wafer is forcibly peeled from the electrostatic chuck by applying a strong force against the residual attracting force, there arises a problem that the wafer may be cracked or undergo an abnormal discharge sufficiently large to destroy devices of the wafer.
To cope with the disadvantages due to a residual attracting force, various methods for eliminating an electric charge stored on an electrostatic chuck have been proposed. For example, a method of eliminating an electric charge stored on an electrostatic chuck upon removal of a sample from the chuck is described in U.S. Pat. No. 5,117,121, wherein a residual attracting force eliminating voltage, having a polarity reversed with respect to that of an attracting voltage and which is higher than the attracting voltage, is applied between the electrodes of the chuck. Another method of eliminating an electric charge stored on an electrostatic chuck is described in Japanese Patent Laid-open No. 185773/1983, wherein a DC voltage for generating an electrostatic attracting force is turned off, and thereafter the radio frequency power for generating a plasma is turned off. Besides these, various methods of removing a sample from an electrostatic chuck are described, for example, in Japanese Patent Laid-open Nos. 112745/1989 and 247639/1992.
The electrostatic chucks described in Japanese Patent Laid-open No. 64950/1982 and Japanese Patent Publication No. 4474/1982 have failed to examine the residual attracting force.
Namely, in the case where the temperature of a wafer as a sample is required to be controlled at a specific value during processing of the wafer, for example, at a plasma processing step, a heat transfer gas is supplied between the back surface of the wafer and the electrostatic chuck. For this purpose, the electrostatic chuck has a structure in which the wafer mounting surface of the chuck is provided with a dispersion groove (called xe2x80x9ca gas groovexe2x80x9d) for uniformly supplying a heat transfer gas. An electrostatic chuck used for holding a wafer subjected to plasma processing is described, for example, in Japanese Patent Laid-open No. 86382/1995, wherein a wafer mounting surface of the chuck has a recess for reducing the contact area between the wafer mounting surface and the wafer, thereby suppressing adhesion of contaminants on the wafer. With respect to the dispersion groove or recess, various patterns have been developed. In the case where a groove or recess is provided in a wafer mounting surface of an electrostatic chuck as described above, the attracting areas on the positive and negative electrode sides change depending on the size and shape of the dispersion groove or recess, so that a residual attracting force is generated.
Further, even in the case where an electrostatic chuck is used during plasma processing the, the amounts of the electric charges stored on attracting surfaces on the positive electrode side and the negative electrode side are different from each other as a result of generation of a self-bias voltage due to the plasma and the application of a radio frequency bias, so that a residual attracting force is generated.
As a result, even the bipolar type electrostatic chuck requires a step of eliminating the electric charge stored on the chuck for removing a residual attracting force, so that there arises a problem in terms of lowering the throughput in carrying wafers. Another problem is that, since an electric charge remains in a dielectric film constituting an attracting surface of the electrostatic chuck, the attracting surface is liable to attract contaminants which in turn adhere on the back surface of a new sample attracted on the attracting surface. In particular, when a wafer is processed using a CVD apparatus which generates deposits having electric charges, such a problem becomes significant.
Further, the method of eliminating a residual attracting force, as described in U.S. Pat. No. 5,117,121, requires the step of eliminating an electric charge stored on the chuck by newly applying a reverse voltage. This causes a problem in terms of lowering the throughput in carrying a sample. Also, if the reverse voltage becomes excessively large, there arises another problem in that an electrostatic attracting force is produced again, to thereby generate a residual attracting force. Besides, in the method of eliminating a residual attracting force, as described in Japanese Patent Laid-open No. 185773/1983, since, after stopping of the supply of the DC voltage for electrostatic attraction, supply of the radio frequency power for generating a plasma is stopped, the time required for eliminating the electric charge must be made longer. This causes a problem in terms of lowering the throughput in carrying a wafer. In the case of supplying a heat transfer gas to the back surface of a sample simultaneously with electrostatic attraction, supply of the transfer gas is usually stopped upon stopping the supply of the DC voltage for electrostatic attraction. As a result, the temperature of the sample is increased and thereby plasma continuous to be produced, so that processing of the sample continues, thereby exerting an adverse effect on the sample, the processing of which should have been terminated.
Additionally, in a plasma processing apparatus, generally, a radio frequency voltage is applied to a sample stage for controlling an incident energy of ions in a plasma against the sample by means of a bias voltage generated at the sample stage. When such a plasma processing apparatus uses a bipolar type electrostatic chuck, it is difficult to equally apply the bias voltage to the sample resulting from the electrode structure of the chuck, as compared with the case of using a monopole type electrostatic chuck. This possibly exerts an adverse effect on uniform processing of the sample.
In view of the foregoing, the present invention has been made, and a first object of the present invention is to provide an electrostatic chuck which is capable of reducing a residual attracting force to a value in a substantially practically usable range.
To achieve the first object, according to the present invention, there is provided an electrostatic chuck including: a pair of electrodes having different polarities; and, a dielectric film formed on top surfaces of the pair of electrodes on which a sample is electrostatically attracted and held when a DC voltage is applied between the pair of electrodes; wherein the amounts of electric charges stored on attracting portions of the dielectric film corresponding to the pair of electrodes, directly before stopping the supply of the DC voltage applied between the pair of electrodes, are substantially equal to each other.
Another object of the present invention is to provide a sample processing method which is capable of reducing the standby time upon removal of a sample from an electrostatic chuck for improving the throughput.
To achieve the second object, according to the present invention, there is provided a sample processing method including the steps of: electrostatically attracting and holding a sample on an electrostatic chuck including a pair of electrodes having different polarities and a dielectric film formed on top surfaces of the pair of electrodes, by applying a DC voltage between the pair of electrodes; and processing the sample electrostatically attracted and held on the chuck through the dielectric film; wherein the amounts of electric charges stored on attracting portions of the dielectric film corresponding to the pair of electrodes, directly before stopping the supply of the DC voltage applied between the pair of electrodes after termination of processing the sample, are substantially equal to each other, so that the electric charges stored on the attracting portions of the dielectric film after stopping the supply of the DC voltage are eliminated due to the balance therebetween, whereby the sample can be removed from the sample mounting surface without addition of any special step.
A further object of the present invention is to provide a sample processing apparatus which is capable of reducing the stand-by time upon removal of a sample from an electrostatic chuck for improving the throughput.
To achieve the third object of the present invention, there is provided a sample processing apparatus for processing a sample electrostatically attracted and held on an electrostatic chuck, the electrostatic chuck including: a pair of electrodes having different polarities; and a dielectric film formed on top surfaces of the pair of electrodes on which a sample is electrostatically attracted and held when a DC voltage is applied between the pair of electrodes; wherein a recess which is not in contact with the back surface of the sample is formed in a surface of the dielectric film on which the sample is disposed; and the amounts of electric charges of different polarities stored on the attracting portions of the surface of the dielectric film, excluding the recess, are equal to each other.